The following disclosure relates to electrical circuits and signal processing.
Power supplies are used to power many types of electronic devices, for example, halogen lamps. Conventional power supplies (e.g., for halogen lamps) typically include a converter. A converter is a power supply switching circuit.
FIG. 1 shows a conventional forward converter 100 including a synchronous rectifier that receives a DC (direct current) or a rectified AC (alternating current) voltage from a power source Vin. Forward converter 100 includes transistors Q1-Q3, inductors L1-L4, a capacitor C1, a clamping diode D1, and a resistor R1. Inductors L3, L4, L1 form a transformer T1. In particular, inductors L3, L4 form primary windings (having opposite polarities) of transformer T1, and inductor L1 forms a secondary winding of transformer T1. Inductor L2 and capacitor C1 form a lowpass LC filter. In operation, during a transformer set period—e.g., when (switching) transistor Q3 turns on—a voltage on the primary windings of transformer T1 is transferred to the secondary winding of transformer T1. During a transformer reset period—e.g., when transistor Q3 turns off—clamping diode D1 turns on to return the voltage set in the secondary winding of transformer T1 to the primary windings of transformer T1.
FIG. 2 illustrates a timing diagram 200 of a voltage Vs of forward converter 100 with respect to time. Voltage Vs represents an unfiltered output voltage of forward converter 100. Ts represents one period cycle for transistor Q3. D1Ts represents a time period during which transistors Q2 and Q3 are on, while transistor Q1 and clamping diode D1 are off. D2Ts represents a time period during which clamping diode D1 and transistor Q1 are on, while transistors Q2 and Q3 are off. D3Ts represents a time period during which clamping diode D1 is off and transistors Q1, Q2, and Q3 are off. The lowpass LC filter filters the high frequency components associated with voltage Vs, and only a DC component of voltage Vs forms an output voltage Vout. As shown in FIG. 2, voltage Vs is equal to (Vin)*(N2/N1) during time period D1Ts, and is equal to zero during time periods D2Ts and D3Ts. An average value for output voltage Vout is therefore given by the following equation:
                              Vout          :=                                    (                              1                Ts                            )                        ·                          [                                                                    Vin                    ·                                          (                                                                        N                          ⁢                                                                                                          ⁢                          2                                                                          N                          ⁢                                                                                                          ⁢                          1                                                                    )                                        ·                    D                                    ⁢                                                                          ⁢                                      1                    ·                    Ts                                                  +                                                      0                    ·                    D                                    ⁢                                                                          ⁢                                      2                    ·                    Ts                                                  +                                                      0                    ·                    D                                    ⁢                                                                          ⁢                                      3                    ·                    Ts                                                              ]                                      ⁢                                  ⁢                              Vout            =                                          (                Vin                )                            *                              (                                  N                  ⁢                                                                          ⁢                                      2                    /                    N                                    ⁢                                                                          ⁢                  1                                )                            *                              (                                  D                  ⁢                                                                          ⁢                  1                                )                                              ,                                    (                  eq          .                                          ⁢          1                )            where N2 represents a number of turns of the secondary winding, N1 represents a number of turns of the primary clamp windings, Vin represents the source voltage, and D1Ts represents the time period during which a switching transistor (e.g., transistor Q3) is on. As shown by equation (1), conventional forward converters typically transfer energy to a secondary winding of a transformer only during an on-time of a switching transistor (e.g., transistor Q3). Conventional forward converters, therefore, generally have a limited efficiency.
In addition to having power converters with limited efficiency, conventional power supplies (e.g., for halogen lamps) typically are not operable to control an output voltage of the power supply, e.g., for dimming control (of output voltage) or for over/under voltage protection. Consequently, if a power supply receives, e.g., a high input voltage, the power supply may pass a high output voltage to an output device (e.g., a halogen bulb), and greatly shorten the life of the output device.